Developmental changes of plant in the regulation of photosynthate distribution of leaves were studied in hydroponically cultivated rice by the 14CO2 tracer technique and analysis of the activity of the regulatory enzymes, sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), and pyruvate kinase (PK). The distribution of primary photosynthates into sugars, amino acids, organic acids, sugar phosphates, proteins, and polysaccharides was determined by column chromatography. The relative primary photosynthate distribution to the sugar phosphate fraction was significantly larger in the 5th leaf than in the 6th one. Correspondingly, the Vmax of PEPC was significantly higher in the 5th than in the 6th leaf, while no significant differences between leaves were detected in the other enzymes. As a consequence, the ratio of the Vmax of SPS and PEPC was lower in the 5th than in the 6th leaf. As the 5th leaf develops before panicle initiation in rice, it predominantly supports vegetative growth, while the 6th leaf develops after panicle initiation and thus contributes mainly to reproductive growth. We conclude that the physiological properties of each leaf are regulated developmentally. When the 6th leaf became fully expanded (corresponding to the panicle initiation stage of plant), the distribution pattern of 14C was transiently changed in the 5th leaf, indicating that individual organs that are mainly involved in vegetative development are affected to some extent by the whole-plant-level physiological transformation that occurs at the transition from the vegetative to the reproductive stage. and T. Shinano ... [et al.].
mRNA expression patterns of genes for metabolic key enzymes sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutamine synthetase 2 were investigated in leaves of rice plants grown at two nitrogen (N) supplies (N0.5, N3.0). The relative gene expression patterns were similar in all leaves except for 9th leaf, in which mRNA levels were generally depressed. Though increased N supply prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS Vmax, SPS limiting and PEPC activities, and carbon flow were examined. The ratio between PEPC activity and SPS Vmax was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5th and 7th leaves) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9th and flag leaves). PEPC activity and SPS Vmax decreased with declining leaf N content. After using 14CO2 the 14C photosynthate distribution in the amino acid fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC/SPS activities ratio, more 14C photosynthate was distributed to the amino acid pool, whereas at higher SPS activity more 14C was channelled into the saccharide fraction. Thus, leaf ontogeny was an important factor controlling photosynthate distribution to the N- or C-pool, respectively, regardless of the leaf N status. and T. Shinano ... [et al.].
Leaf-root interaction is a critical factor for plant growth during maturation and activity of roots is maintained by a sufficient supply of photosynthates. To explain photosynthate distribution among organs in field crops, the node unit hypothesis is proposed. One node unit consists of a leaf and an upper adventitous root, as well as the axillary organs and the lower adventitious root, which is adjacent to one node. Using 14C as tracer, the carbon distribution system has been clarified using spring wheat, soybean, tomato, and potato. The interrelationship among organs from the strongest to the weakest is in the following order: (1) within the node unit > (2) between the node unit in the same or adjacent phyllotaxy > (3) in the main root or apical organs, which are adjacent to the node unit. Within the node unit, 14C assimilated in the leaf on the main stem tended to distribute to axillary organs in the same node unit. The 14C assimilated in the leaf of axillary organs tended to distribute within the axillary organs, including adventitious roots in the axillary organ and then translocated to the leaf on the main leaf of the same node unit. In different organs of the node unit in the same or adjacent phyllotaxy, 14C assimilated in the leaf on the main stem was also distributed to the organs (node unit) belonging to the same phyllotaxy in dicotyledons, while in monocotyledons, the effect of phyllotaxy on 14C distribution was not clear. Among roots/apical organs and node unit, 14C assimilated in the upper node unit was distributed to apical organs and 14C assimilated in the lower node unit was distributed to roots. Thus the node unit hypothesis of photosynthate distribution among organs is very important for understanding the high productivity of field crops. and M. Osaki ... [et al.].
Ontogenetic changes of rates of photon-saturated photosynthesis (Psat) and dark respiration (RD) of individual leaves were examined in relation to nitrogen content (Nc) in rice, winter wheat, maize, soybean, field bean, tomato, potato, and beet. Psat was positively correlated with Nc as follows: Psat = CfNc + Psat0, where Cf and Psat0 are coefficients. The value of Cf was high in maize, medium in rice and soybean, and low in field bean, potato, tomato, and beet, of which difference was not explained by ribulose-1,5-bisphoshate carboxylase/oxygenase (RuBPCO) content. RD was explained by Psat and/or Nc, however, two models must be applied according to plant species. RD related linearly with Psat and Nc in maize, field bean, and potato as follows: RD = a Psat + b, or RD = a'Nc + b', where a, a', b and b' are coefficients. In other species, the RD/Psat ratio increased exponentially with the decrease of Nc as follows: RD/Psat = a exp(b Nc), where a and b are coefficients. Therefore, RD in these crops was expressed as follows: In(RD) = ln(a Psat) + b Nc, indicating that RD in these crops was regulated by both Psat and Nc. and M. Osaki ... [et al.].
In individual leaves, the photon-saturated photosynthetic activity (Psat, expressed on a dry mass basis) was closely related to the nitrogen content (Nc) as follows: Psat = Cf Nc + Psat0, where Cf and Psat0 are constants. On a whole plant basis, the relative growth rate (RGR) was closely related to Nc in canopy leaf as follows: RGR = DMf Nc + RGR0, where DMf and RGR0 are constants. However, the coefficients Cf and DMf were markedly different among plant species. To explain these differences, it is suggested that carbon assimilation (or dry matter production) is controlled by both the Nc in a leaf (or leaves) and by the net N translocation from leaves. This is supported by the finding that Psat is related to the rate of 35S-methionine translocation from leaves. We propose another estimation method for the net N translocation rate (NFR) from leaves: Nc, after full leafing, is expressed as a function of time: Nc = (Nc0 - Ncd) exp(-Nft) + Ncd, where Nf is a coefficient, t is the number of days after leaf emergence, Nc0 is the initial value of Nc, and Ncd is the Nc of the dead leaf. The NFR is then calculated as NFR = ΔNc/Δt = -Nf (Nc - Ncd). Thus Nf is the coefficient for the NFR per unit Nc. NFR is a good indicator of net N translocation from leaves because NFR is closely related to the rate of 35S-methionine translocation from leaves. Since Psat is related to the 14C-photosynthate translocation rate, Cf (or DMf) corresponds to the coefficient of saccharide translocation rate per unit amount of Nc. Cf (or DMf) is closely related to the Nf of individual leaves (or the Nf of canopy leaf). This indicates that C assimilation and C translocation from leaves are related to Nc and N translocation from leaves (net translocation of N). Cf and Nf are negatively correlated with leaf longevity, which is important because a high or low CO2 assimilation rate in leaves is accompanied by a correspondingly high or low N translocation in leaf, and the degree of N translocation in leaves decreases or increases leaf longevity. Thus, since a relatively high Psat (or RGR) is accompanied by a rapid Nc decrease in leaves, it is difficult to maintain a high Psat (or RGR) for a sustained time period. and M. Osaki, T. Shinano.
To find the effects of CO2 enrichment on plant development and photosynthetic capacity of nodulated (line A62-1) and non-nodulated (line A62-2) isogenic lines of soybean (Glycine max Merr.), we examined the interactions among two CO2 treatments (36±3 Pa = AC and 70±5 Pa = EC), and two nitrogen concentrations [0 g(N) m-2(land area) = 0N; 30 g(N) m-2(land area) = 30N]. Nodules were found in both CO2 treatments in 0N of A62-1 where the number and dry mass of nodules increased from AC to EC. While the allocation of dry mass to root and shoot and the amount of N in each organ did not differ between the growth CO2 concentrations, there was larger N allocation to roots in 0N than in 30N for A62-2. The CO2-dependence of net photosynthetic rate
(PN) for A62-1 was unaffected by both CO2 and N treatments. In contrast, the CO2-dependence of PN was lower in 0N than in 30N for A62-2, but it was independent of CO2 treatment. PN per unit N content was unaffected by CO2 concentrations. The leaf area of both soybean lines grown in 30N increased in EC. But in 0N, only the nodulated A62-1 showed an increase in leaf area in EC. Nitrogen use efficiency of plants, NUE [(total dry mass of the plant)/(amount of N accumulated in the plant)] in 30N was unaffected by CO2 treatments. In 0N, NUE in EC was lower than in AC in A62-1, and was higher than that at AC in A62-2. Hence, the larger amount and/or rate of N fixation with the increase of the sink-size of symbiotic microorganisms supplied adequate N to the plant under EC. In EC, N deficiency caused the down-regulation of the soybean plant. and T. Nakamura ... [et al.].